1. Field of the Invention
The present invention relates to an image forming apparatus such as a laser printer or a digital copying machine for forming an image by subjecting a photosensitive member to an electrographic process including charging, exposure, and development.
2. Description of the Related Art
An electrophotographic image forming apparatus includes a charging unit, a laser scanning unit, a developing unit, and a transfer unit, which are for subjecting a photosensitive member of the apparatus to an electrophotographic process. The charging unit uniformly charges a photosensitive surface of the photosensitive member. The laser scanning unit includes a light source and emits laser light according to image data input from an external information unit such as an original reading unit or a computer. The emitted laser light is scanned by a deflection unit (polygon mirror). The scanned laser light is guided through lenses and reflection mirrors to and scanned on the charged photosensitive surface of the photosensitive member. When the photosensitive member is scanned by the laser light, an electrostatic latent image is formed on the photosensitive member. Subsequently, the latent image is developed by the developing unit and transferred by the transfer unit onto a recording sheet.
The laser scanning unit includes a detection device for detecting the laser light scanned by the polygon mirror. The detection device detects the laser light at each scanning of the laser light, and outputs an image-formation start timing signal when detecting the laser light. Based on the image-formation start timing signal and image data, the laser scanning unit outputs the laser light according to an image clock that varies according to image resolution. By emitting the laser light based on the image-formation start timing signal, it is possible to align positions on the photosensitive member where electrostatic latent image portions are formed by respective scannings.
When the temperature and humidity vary in the image forming apparatus, changes occur in positions where component parts (such as the lenses and the reflection mirrors) are installed in the laser scanning unit, causing a change in the position on the photosensitive surface where the electrostatic latent image is formed. The position of the latent image being formed also varies according to the machining accuracy of the component parts of the unit. If the latent image is formed not at a predetermined position, the position on a recording medium where an image is formed is deviated from the desired position. In the case of a color image forming apparatus, scanning lines for respective colors which are to be superimposed on one another are deviated from one another, resulting in degraded image quality.
To obviate this, it is known, as proposed in Japanese Laid-open Patent Publication No. 2003-322810, to adjust the position of a latent image being formed by shifting a pixel clock based on pixel clock phase.
With the proposed technique, however, a plurality of clock generation circuits are provided in order to generate pixel clocks, and therefore the resultant circuit construction becomes complicated. To simplify the circuit construction, it may be possible to adopt a method for performing operations from processing on a laser light detection signal to driving of a laser scanning unit based on a reference clock output from a single oscillator. In that case, however, differences are produced between phases of the reference clock input to respective units due to, e.g., differences between transmission line lengths from the oscillator to the units. The phase differences cause a shift in image writing start position.